Common hazardous materials, including explosives, corrosives and biohazards, are typically solid or liquid. Associated with these materials are volatile (gaseous) chemicals. These volatiles may be the gaseous form of the hazardous material or may be a molecule that derives from a simple chemical reaction of the hazardous material with ambient conditions.
The traditional method of detecting chemical or biological agents, including those associated with hazardous materials, is described with the schematic sketch in FIG. 1. The sketch and discussion of the sketch are summarized from material in Ref. [1]. A conventional sensor system 100 that studies or identifies the chemical components of an analyte (sample) 110 consists of: (1) a chemical detection or recognition element 120, (2) a transducing element 130, and (3) a signal-processing element (including memory) 140. The chemical detection element 120 is composed of some sensing material. It interacts with a sample on a substrate or in the environment and generates a response. The transducer 130 reads the response from the chemical detection element and converts it into a quantifiable signal in a format that can be used as data that are input to the signal processor. The signal processor 140 analyzes the input and determines if a target substance has been detected. It may also quantify the relative amount of the substance. The Readout 150 from the system processor includes an output 160 that is provided to a larger information system or network.
The chemical detection element 120 is the basis of the system. It is characterized by its response, recovery, selectivity and sensitivity. Categories of materials for the chemical detection element include metal and metal oxide semiconductors, solid electrolytes, insulators, catalytic materials, polymers and composites. The detection element may be unique and customized for a given target molecule. The detection element may undergo a structural change or may respond to electric, electrochemical or optical stimulation, and the transducer 130 functions by sensing a change of state associated with one of these responses. The detection element may respond by exhibiting a plurality of different states, either discrete or an analog spectrum of states. The signal processor 140 interprets the information from the transducer to confirm that a change has occurred in the detecting element and identifies the changed state, or identifies which of a plurality of states is present. In such instances the entire system/process of identifying and confirming the presence of the target material is performed by elements distinct from the article (in this case the analyte or substrate) itself. An example of this prior art approach is a stationary (static) system that detects biohazards. In a specific example, the target is anthrax and the article in question (a postal letter or package) is processed by a customized, isolated chamber as the article passes through a mail delivery facility.
System 100 may have multiple and changeable components, but typically is a macroscopic system with a large physical footprint. The signal processor 140 may be a central computing system or a smaller system such as a desktop or laptop computer. The chemical detection element 120 and a matched transducer 130 may be designed for detection of a specific target substance and a variety of these components may be connected to the signal processor.
In the example given above, an article (postal letter or package) serves as a substrate that may or may not include the target analyte and the article is provided to the apparatus in the course of commerce and transactional traffic. In another method of operation, samples are gathered from the field, brought to the system and introduced to the system for analysis. The system apparatus has a large footprint and is relatively expensive. Although the system may have changeable components, the flexibility is severely limited. These are disadvantages of this prior art.
Recent developments in microfabrication have enabled the invention of new kinds of chemical or biological sensor systems. The new system is chip-based, miniaturized, adaptable to different targets and inexpensive. The system builds on the older concept of a system-on-a-chip. A system-on-a-chip may include one or more sensors, digital information processing circuits, memory, and means for input and output of data. The Remote Independent Microsystem (RIM) was disclosed in U.S. Pat. No. 9,432,021. A RIM is micro- or nano-fabricated with dimensions smaller than a traditional system-on-a-chip. It is inexpensive and designed to be expendable after a single use. It operates with very low power and has a long operational lifetime. Detection of chemical or biological agents was mentioned in '021 but this application deserves further consideration.
The new sensor system requires chemical detection elements 120 that are fabricated on micro- or nano-processed chips. The detection elements can be tailored to respond to a specific agent or to a class of agents. Several detection elements with uniquely tailored responses can be fabricated on a single chip. A selection of one element (or of a subset of several element) can be made programmatically by activating the appropriate areas of the chip.
A variety of chemical detection elements are suitable for microfabrication on chips. As one example, there have been many developments in the last two decades concerning the use of carbon nanotubes (CNTs) as detecting elements, typically in the form of a dedicated/integrated sensing/detection portable system. An interaction between a target molecule and a CNT changes the electric resistivity of the CNT. The interaction occurs when the volatile molecule comes in contact with a surface of the CNT. This change of resistive state is the basis of the CNT sensor.
It is not currently practical to fabricate a robust sensor using a single CNT. Therefore an array of CNTs is typically fabricated on top of a pair of interdigitated thin film metal electrodes to form a detecting element. The transducer is a circuit that measures the resistance of the array. The transducer may have the simple form of a current source and voltage detector or the inverse, a voltage source and current detector. The resistance measurement of the transducer is sent to the signal processor where the value is compared with an initial reference resistance value stored in memory. A comparison of the values determines whether an interaction has occurred and the presence of the target molecule can be deduced/confirmed.
An example of a detecting element 200 using CNTs is shown in FIG. 2. Thin film metal wires 220 and 240 form a pair of interdigitated current and voltage electrodes. An array of CNTs 230 is distributed across the surface of the leads and they adhere to the surface. To measure the average resistance of the array, a small current is applied from 220 to 240 and a voltage is measured from 220 to 240. Alternatively, voltage can be applied and current measured. Detector arrays of this kind have been fabricated with lateral dimensions on the order of 10 microns [2].
The process using CNTs is flexible enough that it can be modified in a number of ways in order to detect a variety of target molecules using other forms of chemical interactions. For example, the CNTs can be coated with a thin layer of a chemically active material such as a polymer. An interaction occurs when the volatile molecule comes in contact with the polymer. If the polymer is altered or destroyed by the interaction, the resulting change in the surface state of the CNT causes a corresponding change in resistance. The resistance is measured by the transducer and the process for deducing the presence of the target proceeds in the same way as described above. A different choice of chemical coating results in sensitivity to a different target molecule.
Chemical detection elements based CNTs represent an approach that has successfully demonstrated microfabricated prototypes with dimensions on the order of ten microns or less. Other techniques also are plausible. Traditional detection elements (fabricated with macroscopic dimensions) commonly use polymers. Polymers are organic macromolecules dominantly comprised of carbon and hydrogen atoms, and include heteroatoms such as nitrogen, oxygen, sulfer, phosphorous and etc. as minor constituents. They are characterized by high tailorability, having a broad range of properties and versatility. Both the bulk and surface of a polymer may contain active functional groups which can respond as a chemical sensor. Properties that can be sensed by a transducer include physical and/or chemical (such as mass or volume), electrical (resistivity) and optical.
In addition to these chemical targeted sensors, other environmental condition detecting elements in printed electronic form are known in the art and are sufficiently small (˜4 cm by 8 cm) to be affixed (with an adhesive or similar technique) to packaging or other articles of interest. These elements are sensitive to conditions such as temperature, humidity, pressure, flow or light. As a specific example, in transporting certain pharmaceutical products care must be taken to ensure that they are not exposed to temperatures over a certain threshold. See e.g., U.S. Pat. No. 9,742,466 incorporated by reference herein.
Some additional useful background information for the present invention can be found in: Fundamentals of Sensing Materials. Volume 3: Polymers and Other Materials, edited by Ghenadii Korot Tcenkov, Momentum Press, LLC, New York, N.Y. (2010); Chapter 1, Polymers in Chemical Sensors, B. Adhikari and P. Kar, page 2 and FIG. 1.1; and Li, J., Lu, Y., Ye, Q., Cinke, M., Han, J., and Meyyappan, M., Nano Lett., Vol. 3, p 929 (2003); Li, J. Carbon Nanotubes: Science and Applications, Chapter 9, Editor: M. Meyyappan, CRC Press, Boca Raton, Fla., USA (2004) both of which are incorporated by reference.